Natural arches, stacks and stumps

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The natural arch at Durdle Door, Dorset, Southwest England. — Photo by Saffron Blaze/Wikimedia Commons

AS a youth, I would occasionally cycle 13km from my home in West Cornwall to Land’s End – a granite promontory facing the full force of the Atlantic Ocean. This great headland is aptly named for it is the most south-westerly point in England. Sitting near the cliff’s edge, I would look in awe at the sheer power of the ‘rollers’ as they broke against the cliffs and admire the resistance of a stump of rocks known as The Armed Knight.

Land’s End is part of the West Penwith granite batholith created millions of years ago by tectonic activity when molten granite was intruded into the country rocks. With the subsequent subaerial erosion of these rocks, the granite became exposed to create barren moorlands and spectacular cliff scenery when the sea level later rose. The offshore stump of The Armed Knight still fascinates me even today, I do not know the origin of its name!

Just around the corner from Land’s End there is a cove with a natural arch beneath which lies a steel shipwreck, and this natural arch is the clue as to how The Armed Knight evolved.

A painting of The Armed Knight stump off Land’s End, West Cornwall, England.

Durdle Door in Dorset

This wave crafted natural arch faces the English Channel and lies on the Jurassic Coast World Heritage site as a place of Special Scientific Interest (SSI). It is composed of a resistant limestone, Portland stone, which 170 million years ago, during the Jurassic period, was laid down in shallow warm seas. Seventy million years ago the limestone, made mostly of calcium carbonate, was tectonically uplifted to create a compressed and hardened rock, folded into a dome like structure known by geologists as the ‘Lulworth anticline’.

As the sea level rose at the end of the Ice Ages, about 12 million years ago, the waves began attacking this promontory. The origin of this natural arch is seen in its name where ‘Durdle’ is derived from the Old English ‘thirl’ meaning pierced or bored, and the Middle English words ‘dure’ meaning door or gate. This is exactly what happened here as the waves undercut the cliffs, so caves were excavated in weaknesses in the rock such as joints and bedding planes. Waves become refracted around the end of a promontory and thus can bore into the rock in opposite directions eventually breaking through for the two caves to meet and create a natural arch.

With the passage of time this arch will collapse as its top is subjected to subaerial weathering and the base of the arch by further wave attack and sea spray to create just two pillars or stacks. The force of a breaking wave is determined by the strength of the wind which in turn is affected by the wind’s fetch or the distance of open water over which the wind blows. When breaking on a cliff the wave compresses air in front of it and drives the air into the rock’s inherent structural weaknesses like a wedge. When the wave retreats the compressed air explodes and thus weakens the rock.

Cobra’s Head at Bako National Park

I first saw this famous stack in 1999 when landing at Teluk Assam Beach and wrote about it in thesundaypost of Aug 30, 2015. Sadly, this iconic landing mark is no longer and is no more than a stump after the stack’s total collapse sometime in February this year.

Made of plateau sandstone interbedded with mudstone and pebbles in quite distinct layers in the rock strata at a time in Upper Cretaceous times (75 million years ago), this area was once part of a huge river delta which experienced slumping of its deposits because of rivers in spate and tidal movements.

This slumping is reflected in the angles of the bedding of the rock structure. Much later the area was uplifted by tectonic movements. As the sandstone dried out and contracted so north-westerly trending joints appeared and determined by the alignment of these joints so the South China Sea has exploited these weaknesses in the sandstone over time to create arches, stacks, and eventually stumps.

The Cobra’s Head stack at Teluk Assam Beach, Bako National Park taken in 2022. — Photo by Mark Tan

From the photograph, taken in 2022, it is most likely that this stack was once part of a natural arch which collapsed over time leaving the serpent’s head shaped stack. Further investigation of the photo reveals the wave cut notches and the plane of wave attack between high and low tides. Together with the horizontally bedded limestone it was inevitable that in time this stack would collapse as it did, due to adverse weather conditions which led to huge plunging waves and exceptional rates of erosion. All is not lost on Bako as there are other natural arches and stacks to see with a bit of trekking.

Old Man of Hoy on Orkney Islands, Scotland

This less than 250-year-old, 137-metre-high, sea stack is certainly the most famous one in the UK. In the 1820s it was part of a natural arch, the roof of which collapsed leaving two stacks. Hence its name as it resembles a human figure. An old map of 1600 showed a headland here, which eventually through constant wind and wave attack developed into a natural arch. This area of the Scottish coast receives waves exceeding eight metres per second and strong gales for 30 days each year. It is composed of Old Red sandstone on a plinth of basalt rock.

The Old Red sandstone is not surprisingly red and honey coloured and was formed on desert plains in the Devonian period (380 to 370 million years ago) when Scotland was part of a huge desert continent, south of the Equator. It is separated from the mainland by a 60-metre chasm strewn with a collapsed arch and other cliff debris. Offshore the sea is very deep and thus little of the wave energy is lost to friction with the seabed. As the very resistant rock is horizontally bedded it is susceptible to wave attack and the spray from breaking waves leads to a honeycomb effect on the upper parts of the stack.

It received fame on July 8-9, 1967 on a live BBC outside broadcast entitled ‘The Great Climb’. This expedition involved such famous mountaineers as Chis Bennington, Joe Brown, and Dougal Haston.

It is still climbed today and classified as an extremely severe climb. I watched this climb, and my admiration went to the TV cameramen.

The Apostles along Great Ocean Road, Victoria State, Australia

To achieve this name there must have been 12 stacks here at one time! There are only seven stacks left facing the full force of the Southern Ocean’s waves. It appears that there were nine stacks originally but at the beginning of the 20th century there were eight, and one collapsed in July 2005, now leaving seven.

They are composed of soft Port Campbell limestone containing millions of tiny marine fossils laid down in shallow seas in Mid- to Late-Miocene times (20 to 10 million years ago) later to be uplifted above the plane of wave attack. This limestone is also horizontally bedded, not unlike the Bako sandstone, and thus very vulnerable to wave exploitation.

As with human beings, natural arches, stacks, and stumps have a lifespan. We are all part of the natural world and should take the opportunity to see these wonderful wave creations before, like them, we expire.